The medical conditions of arteriosclerosis and hypertension are potentially debilitating and often life-threatening conditions which require early diagnosis and treatment. These conditions are characterized by changes in arterial blood flow volumes and rates and the response of arterial tissue to changes in blood pressure. A physiological phenomenon which plays a part in these arterial characteristics is referred to herein as arterial compliance, the ability of vasculature to respond to changes in these conditions. The arterial walls of the body include collagen, giving the walls the ability to expand and contract, and muscle tissue which in part controls this expansion and contraction. Vascular compliance includes the response of collagen and muscle in arterial walls to changing conditions. In addition, the condition of arteriosclerosis is characterized by the buildup of fatty substances along arterial walls. These substances can occlude the artery, and can impede the ability of the arterial walls to respond to changing conditions of blood pressure. The fatty substances characteristic of arteriosclerosis are thus a further factor governing arterial compliance. It is thus desirable to be able to analytically understand arterial volume and compliance when diagnosing or treating the medical conditions of hypertension and arteriosclerosis.
An understanding of a patient's arterial volume and compliance is also beneficial when administering anesthesia. The quantity of anesthetic administered to a patient should be just sufficient to eliminate a physiological response by the patient during surgery. If an insufficient amount of anesthetic has been administered, the cardiovascular system will respond reflexively when the patient is intubated prior to surgery. This response can be detected by monitoring arterial volume and compliance, and noting any reduction in these characteristics during intubation. A cardiovascular response can also be detected at the time of the first surgical incision, when an insufficient anesthetic will again be evidenced by a reduction in arterial compliance or volume. Thus, a surgical patient would benefit from the monitoring of arterial volume and compliance by the anesthesiologist and surgeon.
The significance of arterial volume and compliance has been recognized in the prior art. In a sequence of patents including U.S. Pat. Nos. 3,903,872; 4,565,020; 4,651,747; and 4,712,563 issued to William T. Link, methods and apparatus are described for calculating measurements of arterial volume and compliance. Link's technique as described in these patents involves taking and using a series of standard oscillometric blood pressure measurements. The first time derivative of the measured cuff pressure pulse, dP, at a time within a patient's actual blood pressure pulse as a function of applied cuff pressure is then calculated to inferentially determine arterial volumetric changes dV. As Link shows, this first derivative corresponds to changes in arterial volume changes dV. A curve plotted from these calculations is transformed by Link to a curve of volumetric change as a function of transmural pressure, V/P, and this curve may in turn be differentiated to obtain a compliance curve dV/dP.
In a second sequence of patents including U.S. Pat. Nos. 4,664,126; 4,697,596; 4,699,151; and 4,699,152, Link extends this analysis to a technique in which the peak to peak amplitude of each cuff pulse and the patient's diastolic and systolic pressures are used to calculate a particular patient's own volumetric and compliance curves. Again, the volumetric and pressure information is determined inferentially from arterial pressure pulse information. The volumetric and pressure curves are used by Link in the determination of systolic, diastolic, and mean arterial blood pressure. The Link technique utilizes a ramp-up method of measuring pressure pulses, wherein pulse data is taken during inflation of a blood pressure cuff. The currently preferred technique for taking such measurements, which is incrementally deflating a blood pressure cuff from a pressure level in excess of systolic pressure and taking measurements over a range of declining pressure steps, is described in U.S. Pat. Nos. 4,349,034 and 4,360,029, issued to Maynard Ramsey, III.
A display of information concerning arterial volume which is useful to the anesthesiologist, surgeon or diagnostician is a curve representing arterial volume (in cc) or area (in mm.sup.2) as a function of transmural pressure (in mm Hg). At a given point on the positive pressure side of this curve the volume or area may be represented by a value R, the effective arterial radius. The slope of the curve at any given point, dV/dP, represents arterial compliance, and a plot of dV/dP as a function of transmural pressure represents the arterial compliance curve.
In accordance with the principles of a parent of this invention, Ser. No. 453,919, now U.S. Pat. No. 5,103,833, the patient's arterial volume and compliance is represented in this format and, in correspondence thereto, the value of R over time is calculated and displayed. The display of this data provides the anesthesiologist with information concerning the patient's arterial volume and compliance characteristics, and also provides information as to changes occurring in arterial volume over time. This will enable the anesthesiologist to detect any response of the cardiovascular system to intubation or incision during a surgical procedure, thereby facilitating the correct delivery of anesthetic to the patient.
A display as described above may be further enhanced by providing the arterial compliance dV/dP at a given transmural pressure for a patient undergoing diagnosis or monitoring. The maximum value of dV/dP, referred to as peak arterial compliance, can also be ascertained from this information. A further display of this information which would be of use to a clinician would be a representation of arterial capacity, R, in relation to the radius of the limb at which the blood pressure cuff of the monitoring instrument is attached.
In accordance with another aspect of the aforementioned patent, a variation of this display format provides a display of the patient's arterial volume data prior to the initiation of any surgical intervention and, in correspondence therewith, a current display of arterial volume data as the surgical intervention proceeds. Comparison of the data informs the anesthesiologist of the cardiovascular system response to bodily stimuli during the procedure.
A recent proposal relating to the determination of vascular compliance is known as the "Hartsafe Product Concept." This product concept is further described in Raines, Jaffrin and Rao, "A Noninvasive Pressure-Pulse Recorder: Development and Rationale" Medical Instrumentation, Vol. 4, September-October 1973, pp. 245-250. In this procedure, a pressure cuff is strapped to a patient's calf and inflated. When the cuff pressure attains a level of 70 mm Hg, a calibration step is initiated by injecting one ml of air into the cuff. The system measures the change in pressure resulting from this quantified injection and calculates a calibration factor based upon the change. Cuff inflation continues and volume pulse signals are recorded until a minimal volume pulse signal or a maximum pressure value of 225 mm Hg is attained. The system then commences a step deflate sequence. At individual pressure decrement steps of 10 mm Hg the volume pulse signal is recorded. The sequence continues until a minimal pressure level is attained, at which time data acquisition is complete. The system then performs "signal conditioning" using the volume pulse and cuff pressure signals at each 10 mm Hg cuff pressure decrement, and the calibrate signal previously stored. The volume-pressure curve, peak compliance, and other parameters are obtained by this "signal conditioning." The Raines/"Hartsafe" approach seems to be a more direct measurement of arterial volume than the Link techniques, in which arterial volume is calculated premised upon its relationship to the arterial pressure pulses, because an actual measurement of system response to a known change in cuff volume is taken during the calibration step of "Hartsafe." But the actual data which is computed for the volume-pressure curve appears to be similarly inferential, however, as the single calibration volume measurement is the only volumetric measure used in conjunction with the pulse signals to inferentially calculate the curve.
It would be desirable to provide arterial volume and compliance information that is based upon direct measurements of arterial volume and changes in arterial volume. It would further be desirable to continually recalibrate the system during the acquisition of such volumetric data, or to obviate the need for calibration entirely by obtaining highly accurate volumetric data in the first instance. In accordance with a further aspect of Apple, U.S. Pat. No. 5,103,833, a system for measuring arterial cross-section and compliance is provided in which a pressure cuff on a peripheral part of the body is inflated to a pressure level which occludes arterial vessels. The cuff is then deflated, and pressure measurements taken in correspondence with decreasing pressure levels. Air which is expelled during deflation is removed from the cuff through means for determining the volume of air expelled. This means may comprise, for instance, an orifice or transfer volume of known characteristics, or a flow measurement device. At each point at which a pressure determination is made, the volume of air removed from the cuff is precisely known, or is calculated based upon an immediately obtained volumetric calibration. Thus, there is no need for the use of a calibration factor or reliance upon a single prior calibration step in the determination of arterial volume and compliance performed by the system.
As air is removed from the pressure cuff in accordance with the Apple invention, the oscillation pressure peaks and changes in cuff volume as a function of pressure are recorded over a range of cuff pressures. From this information the oscillation volume is calculated. From the knowledge of oscillation volume measurements over the range of cuff pressures and conventional determination of systolic and diastolic pressure levels, the patient's arterial volume and compliance curves are reconstructed. Thus, accurate and complete information concerning blood pressure and arterial volume, cross-section and compliance in relation to transmural pressure and/or time is provided to the physician for monitoring and diagnosis.